Currently, roblox-ts transpiles the modulo operator directly to a Lua modulo operator. This is an issue as in Lua, the modulo operator will always return a positive value, while TypeScript will return a negative value if the dividend is negative.

Consider the following assertions (copied from the MDN web docs)
https://roblox-ts.com/playground/#code/IYZxFMCcBcAoEYDMACApMgrMgvL5iBKAbgChQIZYBaJNTHPKw08qOAFjoCYHtkAGYmTBtqndD1x9BpIA

This will pass when ran in TypeScript, but fail when ran through roblox-ts.

You can check this right now by running the following snippets:

TypeScript/JavaScript (run in the Inspect Element console window)

console.assert(13 % 5 === 3);
console.assert(-13 % 5 === -3);
console.assert(4 % 2 === 0);
console.assert(-4 % 2 === 0);

Lua (run in the Roblox command line)

assert(13 % 5 == 3);
assert(-13 % 5 == -3);
assert(4 % 2 == 0);
assert(-4 % 2 == 0);

This seems unintentional, as roblox-ts takes measures to ensure parity with TypeScript in the case of things such as array indices starting from 0, where they start from 1 in Lua.